The most diverse, abundant, and widely distributed tissue; functions: bind, support, and protect organs; acts as the glue that secures body structures together.
Examples: tendons (muscle to bone), ligaments (bone to bone), adipose tissue, cartilage, bone, and blood.
Common origin: all derived from embryonic mesenchyme.
Vascularity varies by type; regenerative capacity also varies: bone and blood are highly regenerative; cartilage is not.
Three Components of Connective Tissue (ECM)
Three basic components: cells, protein fibers, and ground substance; together form the extracellular matrix (ECM).
Diversity arises from different types/amounts of fibers and ground substance.
Classification of Connective Tissue
Three broad categories: 3 main groups:
Connective tissue proper
Supporting connective tissue
Fluid connective tissue
Connective tissue proper subdivides into loose (fewer fibers, more ground substance) and dense (more fibers, less ground substance):
Loose: Areolar, Adipose, Reticular
Dense: Regular, Irregular, Elastic
Supporting connective tissue includes:
Cartilage (Hyaline, Fibrocartilage, Elastic)
Bone (Compact, Spongy)
Fluid connective tissue: Blood
Connective Tissue Proper: Cells
Resident cells (permanently housed):
Fibroblasts: produce fibers and ground substance
Adipocytes: fat cells; clusters form adipose tissue when dominant
Mesenchymal cells: embryonic stem cells; can differentiate into needed connective tissue cells; act as repair stem cells in adults
Wandering cells: continuously move through tissue; mainly leukocytes and related immune cells; aid in defense and repair (e.g., dendritic cells in dermis)
Protein Fibers
Collagen fibers: unbranched, cablelike, very strong and flexible; resist stretching; ~25\% of body protein; appear white; pink in H&E; abundant in tendons/ligaments
Reticular fibers: thinner than collagen; same subunits as collagen; form branching networks; provide structural framework in organs like lymph nodes, spleen, liver
Elastic fibers: contain elastin; branch and recoil; yellow fibers; visible with specialized stains; abundant in skin, arteries, and lungs
Ground Substance
Ground substance is produced by connective tissue cells; fills ECM alongside fibers; may be viscous, semisolid, or solid depending on tissue
Contains large molecules:
Glycosaminoglycans (GAGs): ext{glycosaminoglycans}; negatively charged and hydrophilic; attract cations (e.g., Na^+) and water, increasing viscosity
Types of GAGs: chondroitin sulfate, heparan sulfate, hyaluronic acid
Proteoglycans: GAGs linked to a protein core
Adherent glycoproteins: glue that helps attach cells and fibers to ground substance
Ground substance plus fibers form the dynamic extracellular matrix, through which cells and molecules move
Embryonic Connective Tissue
Two types in the embryo: Mesenchyme and Mucous (Wharton’s jelly) connective tissue
Mesenchyme: origin of all connective tissues; contains mesenchymal cells and immature protein fibers in a gel-like ground substance
Adult mesenchymal cells act as stem cells to support tissue repair and differentiation into required connective tissue cells